Does .NET 3.5 (WCF, or other) offer any type of push notification service?
I have a server and a client. I want a service to run on the server and if x happens
You don't need to use any call backs. You can use MSMQ with WCF. You can use the attached example I have made. If you sent it to be a public push server - i.e available via WebService - you will have to add a webservice that will use the MSMQ. The problem is that you will need to have an IIS installed on both the server and the client. If you use it internally then MSMQ will be enough. http://www.codeproject.com/KB/WCF/WcfMsmqServerPush.aspx
Your question couldn't be any more general, but I'm going to take a guess and suggest you look at WCF callbacks.
I doubt your question relates to WPF or even WF, but you will need to elaborate if you think otherwise.